for enabling/disabling v2 control flow.
*   Enable v2 control flow as part of `tf.enable_v2_behavior()` and
    `TF2_BEHAVIOR=1`.
*   Fixes autocomplete for most TensorFlow API references by switching to use
    relative imports in API `__init__.py` files.
*   Auto Mixed-Precision graph optimizer simplifies converting models to
    `float16` for acceleration on Volta and Turing Tensor Cores. This feature